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Commit131cd95

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Initial attempt at approximation of catamorphism using fixed inductive types
1 parent1760ec4 commit131cd95

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7 files changed

+168
-9
lines changed

7 files changed

+168
-9
lines changed
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packagecom.jnape.palatable.lambda.recursionschemes;
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importcom.jnape.palatable.lambda.functions.Fn1;
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importcom.jnape.palatable.lambda.functor.Functor;
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publicinterfaceAlgebra<FextendsFunctor<A, ?>,A>extendsFn1<F,A> {
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}
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packagecom.jnape.palatable.lambda.recursionschemes.builtin;
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importcom.jnape.palatable.lambda.functions.Fn1;
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importcom.jnape.palatable.lambda.functions.Fn2;
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importcom.jnape.palatable.lambda.functor.Functor;
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importcom.jnape.palatable.lambda.recursionschemes.Algebra;
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importcom.jnape.palatable.lambda.recursionschemes.Fix;
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publicclassCatamorphism<A,FextendsFunctor,FAextendsFunctor<A,F>>implementsFn2<Algebra<FA,A>,Fix<F, ?extendsFunctor<?extendsFix<F, ?>,F>>,A> {
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privatestaticfinalCatamorphismINSTANCE =newCatamorphism();
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@Override
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@SuppressWarnings("unchecked")
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publicAapply(Algebra<FA,A>algebra,Fix<F, ?extendsFunctor<?extendsFix<F, ?>,F>>fixed) {
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returnalgebra.apply((FA)fixed.unfix().fmap(x ->cata(algebra, (Fix<F,Functor<Fix<F, ?>,F>>)x)));
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}
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@SuppressWarnings("unchecked")
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publicstatic <A,FextendsFunctor,FAextendsFunctor<A,F>>Catamorphism<A,F,FA>cata() {
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returnINSTANCE;
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}
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publicstatic <A,FextendsFunctor,FAextendsFunctor<A,F>>Fn1<Fix<F, ?extendsFunctor<?extendsFix<F, ?>,F>>,A>cata(
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Algebra<FA,A>algebra) {
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returnCatamorphism.<A,F,FA>cata().apply(algebra);
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}
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publicstatic <A,FextendsFunctor,FAextendsFunctor<A,F>>Acata(
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Algebra<FA,A>algebra,Fix<F, ?extendsFunctor<?extendsFix<F, ?>,F>>fixed) {
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returncata(algebra).apply(fixed);
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}
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}
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packagecom.jnape.palatable.lambda.recursionschemes.builtin;
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importcom.jnape.palatable.lambda.recursionschemes.Algebra;
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importorg.junit.Test;
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importtestsupport.recursion.ListF;
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importtestsupport.recursion.NatF;
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importstaticcom.jnape.palatable.lambda.recursionschemes.Fix.fix;
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importstaticcom.jnape.palatable.lambda.recursionschemes.builtin.Catamorphism.cata;
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importstaticorg.junit.Assert.assertEquals;
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importstatictestsupport.recursion.List.cons;
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importstatictestsupport.recursion.List.nil;
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importstatictestsupport.recursion.Nat.s;
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importstatictestsupport.recursion.Nat.z;
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publicclassCatamorphismTest {
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@Test
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publicvoidfoldingThroughLeastFixedPoint() {
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Algebra<NatF<Integer>,Integer>sum =nat ->nat.match(z ->0,s ->s.carrier() +1);
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assertEquals((Integer)0,cata(sum).apply(fix(NatF.z())));
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assertEquals((Integer)0,cata(sum).apply(z()));
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assertEquals((Integer)3,cata(sum).apply(fix(NatF.s(fix(NatF.s(fix(NatF.s(fix(NatF.z())))))))));
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assertEquals((Integer)3,cata(sum).apply(s(s(s(z())))));
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Algebra<ListF<String,Integer>,Integer>length =list ->list.match(nil ->0,cons ->1 +cons.tail());
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assertEquals((Integer)0,cata(length).apply(fix(ListF.nil())));
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assertEquals((Integer)0,cata(length).apply(nil()));
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assertEquals((Integer)3,cata(length).apply(fix(ListF.cons("3",fix(ListF.cons("2",fix(ListF.cons("1",fix(ListF.nil())))))))));
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assertEquals((Integer)3,cata(length).apply(cons("3",cons("2",cons("1",nil())))));
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}
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}
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packagetestsupport.recursion;
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importcom.jnape.palatable.lambda.recursionschemes.Fix;
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publicfinalclassList<A>implementsFix<ListF<A, ?>,ListF<A,List<A>>> {
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privatefinalListF<A,List<A>>unfixed;
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privateList(ListF<A,List<A>>unfixed) {
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this.unfixed =unfixed;
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}
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@Override
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publicListF<A,List<A>>unfix() {
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returnunfixed;
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}
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publicstatic <A>List<A>nil() {
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returnnewList<>(ListF.nil());
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}
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publicstatic <A>List<A>cons(Ahead,List<A>tail) {
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returnnewList<>(ListF.cons(head,tail));
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}
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}
Lines changed: 33 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1,18 +1,18 @@
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packagetestsupport.recursion;
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importcom.jnape.palatable.lambda.adt.coproduct.CoProduct2;
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importcom.jnape.palatable.lambda.functor.Functor;
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importjava.util.function.Function;
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publicabstractclassListF<A,B>implementsFunctor<B,ListF<A, ?>> {
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publicabstractclassListF<A,B>implementsFunctor<B,ListF<A, ?>>,CoProduct2<ListF.Nil<A,B>,ListF.Cons<A,B>> {
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publicstatic <A,B>ListF<A,B>nil() {
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returnnewNil<>();
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}
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publicstatic <A,B>ListF<A,B>cons(@SuppressWarnings("unused")Aa,
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@SuppressWarnings("unused")Bb) {
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returnnewCons<>();
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publicstatic <A,B>ListF<A,B>cons(Ahead,Btail) {
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returnnewCons<>(head,tail);
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}
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publicstaticfinalclassNil<A,B>extendsListF<A,B> {
@@ -22,14 +22,42 @@ public static final class Nil<A, B> extends ListF<A, B> {
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public <C>ListF<A,C>fmap(Function<?superB, ?extendsC>fn) {
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return (Nil<A,C>)this;
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}
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@Override
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public <R>Rmatch(Function<?superNil<A,B>, ?extendsR>aFn,
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Function<?superCons<A,B>, ?extendsR>bFn) {
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returnaFn.apply(this);
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}
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}
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publicstaticfinalclassCons<A,B>extendsListF<A,B> {
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privatefinalAhead;
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privatefinalBtail;
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privateCons(Ahead,Btail) {
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this.head =head;
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this.tail =tail;
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}
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publicAhead() {
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returnhead;
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}
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publicBtail() {
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returntail;
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}
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@Override
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@SuppressWarnings("unchecked")
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public <C>ListF<A,C>fmap(Function<?superB, ?extendsC>fn) {
32-
return (ListF<A,C>)this;
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returnnewCons<>(head,fn.apply(tail));
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}
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@Override
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public <R>Rmatch(Function<?superNil<A,B>, ?extendsR>aFn,
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Function<?superCons<A,B>, ?extendsR>bFn) {
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returnbFn.apply(this);
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}
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}
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}
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packagetestsupport.recursion;
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importcom.jnape.palatable.lambda.recursionschemes.Fix;
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publicfinalclassNatimplementsFix<NatF,NatF<Nat>> {
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privatefinalNatF<Nat>carrier;
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privateNat(NatF<Nat>carrier) {
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this.carrier =carrier;
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}
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@Override
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publicNatF<Nat>unfix() {
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returncarrier;
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}
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publicstaticNatz() {
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returnnewNat(NatF.z());
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}
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publicstaticNats(Natn) {
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returnnewNat(NatF.s(n));
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}
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}

‎src/test/java/testsupport/recursion/NatF.java

Lines changed: 11 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -11,8 +11,8 @@ public static <A> NatF<A> z() {
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returnnewZ<>();
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}
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publicstatic <A>NatF<A>s(@SuppressWarnings("unused")Aa) {
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returnnewS<>();
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publicstatic <A>NatF<A>s(Aa) {
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returnnewS<>(a);
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}
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publicstaticfinalclassZ<A>extendsNatF<A> {
@@ -33,13 +33,20 @@ public <R> R match(Function<? super Z<A>, ? extends R> aFn, Function<? super S<A
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}
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publicstaticfinalclassS<A>extendsNatF<A> {
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privateS() {
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privatefinalAa;
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privateS(Aa) {
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this.a =a;
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}
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publicAcarrier() {
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returna;
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}
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@Override
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@SuppressWarnings("unchecked")
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public <B>NatF<B>fmap(Function<?superA, ?extendsB>fn) {
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return(NatF<B>)this;
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returnnewS<>(fn.apply(a));
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}
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@Override

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